1. Field of the Invention
This invention relates to a converter circuit for a polyphase switched inductive load. The invention is particularly, though not exclusively, applicable to a converter circuit for a switched reluctance machine.
2. Description of Related Art
FIG. 1 illustrates the basic elements of a typical four-phase switched reluctance machine. It has eight salient poles on the stator and six salient poles on the rotor. Both the stator and the rotor are laminated. Each stator pole carries a simple exciting coil, and opposite coils are connected to form the north and south poles of one `phase`. Only one phase winding is shown for the sake of clarity, excited from a d.c. supply.
Torque is developed by the tendency of the magnetic circuit to adopt a configuration of minimum reluctance, i.e., for a pair of rotor poles to be pulled into alignment with an excited pair of stator poles, maximizing the inductance of the exciting coils. Continuous rotation (in either direction) is achieved by switching the phases in the appropriate sequence, so that torque is developed continuously in the desired direction. The larger the current supplied to the coils, the greater the torque.
When a pair of rotor poles are fully aligned with a pair of stator poles, the effective air gap between them is at a minimum and the inductance is at a maximum so that the flux linking the coils is also at a maximum for a given current. Excitation of the winding coils creates the magnetic forces that tend to attract the rotor to this orientation of minimum reluctance. By timing and sequencing the switching of each phase as the rotor poles approach pairs of stator poles, continuous rotation of the rotor can be achieved.
Similarly, the same switched reluctance machine can be arranged as a generator by mechanically driving the rotor past the stator poles and timing and sequencing the switching of the coils as the rotor poles are leaving the position of minimum reluctance so that the mechanical work on the rotor is converted into a current flowing in the windings.
A conventional converter circuit is designed to receive a d.c. or rectified mains a.c. supply and to apply it to a load according to a switching strategy. Converters for switched reluctance machines are described in the paper `The Characteristics, Design and Applications of Switched Reluctance Motors and Drive` by J. M. Stephenson and R. J. Blake delivered at PCIM '93 at Nurnberg, Germany, Jun. 21-24, 1993. This paper discloses the typical two-switches-per-phase circuit shown in FIG. 1. In this kind of converter circuit, the voltage drop across both of the switches is significant when the supply voltage is low, e.g., around 12 volts as would be found in an automotive electrical system.
To reduce the number of switches and yet maintain efficient energy conversion, the so-called `C-dump` circuit was developed. This uses n+1 switches, where n is the number of phases of the load switched by the converter. One form of the C-dump circuit is illustrated in FIG. 2. The circuit operates by "dumping" stored magnetic energy at the end of a switching cycle into a capacitor (C.sub.0 in FIG. 2) which operates at a voltage above the main dc link. To regulate this voltage, a down converter is required (L.sub.0 /D.sub.0 /T.sub.0 in FIG. 2) to return energy to the dc link. It will be appreciated that, even though the C-dump circuit realizes a reduction in the component count over the two-switches-per-phase converter there is still an inductor/diode/switch arrangement that does not contribute to the motor switching function but is essential to the circuit operation as the means to return electrical energy back to the supply.
The C-dump converter circuit and other converter topologies are described in the paper `A Modified C-Dump Converter for Variable-Reluctance Machines` by A. Hava, V. Blasko and T. A. Lipo, IEEE Transactions on Industry Applications, Vol. 28, No. 5, Sept/Oct 1992, and in the paper `SRM Inverter Topologies: A Comparative Evaluation` by S. Vukosavic & V. R. Stefanovic, IEEE Trans on Ind. Appl., Vol. 127, No. 6, 1991, p. 1034-47.
In a polyphase switched reluctance machine for a mass-produced article, such as a washing machine, the cost of the switching devices is significant. Therefore, if any further saving on the component count in the converter circuit can be realized, significant cost and space benefits may be achieved.